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Discussion Starter #1
I am helping a friend do a 2wd to 4wd swap and we are putting the entire driveline in. The truck we are swapping into has a 2 piece exhaust manifold with the twin cats, 4 oxygen sensors. The donor has the older style setup with a single cat near the muffler and a pair of oxygen sensors.

We want to use the older distributor engine as it only has 30k miles and the better head.

So I know we can is the adapter plate to mate the newer exhaust to the older head but its unavailable right now.

So...and keep in mind we don't have to pass emissions but we don't want the MIL coming on.

Could I just simply weld new sensor bungs in the same exact location upstream? The manifold has a central location right before the collector where I could get Bank 1 and 2 to work.

Then I could just use O2 simulators for the downstream sensors.

Thoughts?
 

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I've gone the opposite way, used a 331 head with older exhaust manifold. Just needed to use the matching manifold gasket. Not 100% sure it will go together using the old head with the newer exhaust manifolds, but I think it will. The OEM style manifold gasket is MLS steel and should fill any small gaps. Bolt pattern is identical.

One thing with the older head is that it does not have the bolt bungs for the in line coil. You can probably swap that for a Mopar V6 minivan coil, and use regular wires with it. just have to verify the connector pinout will match the firing order. The terminals can be swapped around in the connector if needed. I believe the connector will plug in the same physically.

The late 4.0l JTEC PCM is too smart for most bypass solutions on the downstream O2 sensors, they won't pass the warm-up self tests, usually. They will want to be downstream of two pre-cats in the 4 sensor CA spec pcm. Fed spec PCMs don't have pre-cats, that might be a solution, easy to weld a bung behind a single cat for them.

Hope this helps!
 

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Thanks guys. We ended up using the steel gasket with the newer exhaust manifold. It is working so far.

Yeah, lots of little stuff to deal with. He can fab some coil brackets. My work is done 😁
 
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